The invention relates to a transmitter amplifier in which a transistor is provided as a class B or C amplifier element.
Transmitter amplifiers, which operate in class B or C and are equipped with a transistor, include, at the input and output of the transistor, networks, which serve for the transformation of the signal source resistance at the input of the transistor and for the transformation of the output power at the load resistor or at the antenna. Part of the elements of these networks are additionally used for introducing the operational voltage and adjustment of the operating point. These networks comprise reactive elements, capacitors and inductors, so that the transformation paths are frequency-dependent and give the required transformation only for the desired frequency or for the desired frequency band. Further the requirement that the transmitter amplifiers operate stably even in the case of a mismatch at the input and output and produce no parasitic oscillations, is placed on transmitter amplifiers.
In practice, however, in the case of mismatching of the output, which, for example, is realized by an open or short-circuited line of variable length, the excitation of parasitic frequencies f.sub.p outside the desired frequency band is observed. The parasitic oscillations are of a substantially lower frequency than the desired frequency, their mixing products with the desired frequency f.sub.N however yield frequencies f.sub.M which fall into the desired frequency band, since EQU f.sub.M = f.sub.N .+-. nf.sub.p
wherein integral multiples of the parasitic frequency are designated by n (= 1, 2, 3 and so on).
To suppress these parasitic frequencies in the input and output of the stage, it is known to insert frequency-dependent elements, which represent damping for the low frequencies. The success of these measures is only slight on account of the specified frequency dependency, because, on the one hand the damping cannot be chosen to be any effecting level, without reducing the amplification of the desired frequency, and, on the other hand, the damping is not designed to be broad banded at any event, so that this does not act in like manner for all the frequencies lying far outside the desired frequency band.